A frequently encountered situation is that of considering the optimum placement of electronic equipment within a particular region of interest and to minimize the adverse affect of external interference electromagnetic fields. Also, a further consideration is the determination of what the associated shielding requirements should be in this same region for specified equipment.
If only radiation fields are present, then standard antenna measurements may be taken in order to map the field in the region of interest from which necessary information as to field strength, polarization and Poynting vector can be obtained. Although an antenna is not strictly speaking a "free field sensor" since it perturbs the field in the region of measurement, it perturbs the field in a way that enables determining what the field would be without the antenna present. Moreover, in purely radiation fields, the electric and magnetic vectors are uniquely related so that basic determinations can be made solely in terms of either the electric or the magnetic field, this typically being accomplished by electric field measurements. With this information in hand, theoretical assessment of the effect of placing any object (e.g., electronic equipment) in the region of interest is determinable since radiation fields imply a known boundary value condition at infinity.
The more usual and more difficult situation, however, is to effect measurements of reactive fields. To determine the reactive field prior to the placement of certain, say, electronics equipment in the region of interest, a satisfactory field sensor must be able (1) not to appreciably alter the field being measured and (2) the sensor must measure all six field components and their relative phases. By way of comparison, in a radiation field situation it can be safely assumed that the electronic equipment shall not have significant interaction with the electromagnetic field source whereas, however, this will not be true in the usual reactive field case. In either situation, the topology of neighboring metal or conductive boundaries must be taken into account.
As a practical example, modern day aircraft not only have electronic gear of great variety, but also are exposed to external interference electromagnetic fields which can possess relatively high intensities (e.g., radar). Moreover, since the external construction of the aircraft includes many metal features in complex arrangements, it is desirable and difficult to determine the best location for such gear from the standpoint of protecting the gear against electromagnetic field pollution. However, because complex metal topology of the reactive fields are exceedingly complex and not readily predictable, a means of measuring fields, both reactive and radiative, at any particular location quickly, easily and inexpensively would be highly advantageous.